CAMP PENDLETON, Calif. — In the early morning along a barren stretch of beach here last week, Japanese soldiers and American Marines practiced how to invade and retake an island captured by hostile forces.

Memo to Beijing: Be forewarned.

One Marine sergeant yelled for his men, guns drawn, to push into the right building as they climbed through the window of an empty house meant to simulate a seaside dwelling.The Marines had poured out of four amphibious assault vehicles as another group of smaller inflatable boats carrying soldiers of Japan’s Western Army Infantry Regiment landed in an accompanying beachhead assault.

There were shouts in Japanese. There were shouts in Marine English.There was air support, from Huey and Cobra helicopters hovering above. Then larger Navy hovercrafts roared in, spitting up a spray of seawater before burping out Humvees and more Japanese troops, their faces blackened with camouflage paint.

American military officials, viewing the cooperative action of the former World War II enemies from a nearby hillside, insisted that the annual exercise, called Iron Fist, had nothing, nothing to do with last fall’s game of chickenbetween Tokyo and Beijing over islands that are largely piles of rocks in the East China Sea. But Lt. Col. John O’Neal, commander of the 15th Marine Expeditionary Unit, said that this year, the Japanese team came with “a new sense of purpose.”

“There are certainly current events that have added emphasis to this exercise,” he said, as Japanese soldiers made their way up into the rocks before disappearing into the hills above the beach. “Is there a heightened awareness? Yes.”

Image

The Japanese arrived nearly 250 strong as tensions with Beijing over a set of disputed islands in the East China Sea continue.CreditJoe Klamar/Agence France-Presse — Getty Images

In the United States military, commanders are increasingly allied in alarm with Japan over China’s flexing of military muscle. Capt. James Fanell, director of intelligence and information operations with the United States Pacific Fleet, recently said in San Diego that China was training its forces to be capable of carrying out a “short, sharp” war with Japan in the East China Sea.

In a sign of continuing concern, Gen. Ray Odierno, the Army chief of staff, was in China over the weekend seeking to improve the limited relationship between the American and Chinese militaries, perhaps through exchanges of top officers. In recent years, the Pentagon has worried about the buildup of China’s military and a lack of transparency among its leaders.

The islands at the center of the dispute, known as the Senkaku in Japanese and the Diaoyu in Chinese, are a seven-hour boat ride from Japan, even farther from China, and thought to be surrounded by man-eating sharks. Japan has long administered the islands, but they are claimed by China and Taiwan.

Last year, China set off a trans-Pacific uproar when it declared that an “air defense identification zone” gave it the right to identify and possibly take military action against aircraft near the islands. Japan refused to recognize China’s claim, and the United States defied China by sending military planes into the zone unannounced — even as the Obama administration advised American commercial airlines to comply with China’s demand and notify Beijing in advance of flights through the area.

A few weeks later, Prime Minister Shinzo Abe of Japan approved a five-year defense plan that took the pacifist nation further toward its most assertive military posture since World War II.

This year, when Japanese troops showed up for the exercise with the Marines at Camp Pendleton, they came packing. Instead of the platoon of 25 soldiers they sent to the exercise in 2006, the first year it was conducted, the Japanese arrived nearly 250 strong.They brought along their own Humvees, gear and paraphernalia for retaking islands — or, in Marine parlance, “amphibious assault with the intent to seize objectives inland.”

The monthlong exercise, which ends Monday, has been spread over a wide section of Southern California. There was the amphibious assault at Camp Pendleton, mortar shoots at the Marine Corps Air Ground Combat Center at Twentynine Palms and live firing exercises at San Clemente Island. There was a nighttime raid at Naval Amphibious Base Coronado, presumably out of sight of guests sipping pink Champagne on the verandas of the bejeweled Hotel del Coronado a short distance away.

This year’s Iron Fist, Colonel O’Neal said, was the largest and most involved operation so far. The exercise included drones and the kinds of air support that would be needed to protect Japanese and American troops retaking an island, though the “shaping” that would normally be done in a real-world assault — when the Air Force and Navy bomb intended targets before carrying out an actual ground invasion — was only implied.

In the waters just off Coronado last month, Japanese soldiers, clutching their gear, pushed rubberreconnaissance boats out of a hovering helicopter and jumped into the cold water as part of what the Marines called “helo cast” training. The bread and butter of the Marine Corps, helo cast training, with its emphasis on fast and light movements into hostile territory, is not the type of training which Japanese troops have routinely had in the past.

The Japanese soldiers and the Marines have spent much of the past month managing a considerable language barrier. Although they have worked side by side in the joint exercises, they are not intertwined, hence the reason for the parallel amphibious landings. Marine interpreters and their Japanese counterparts dashed between the two militaries, discussing coordinates and plans.

Image

American military officials insisted that the annual exercise was unrelated to the disputebetween Tokyo and Beijing over islands in the East China Sea.CreditT. Lynne Pixley for The New York Times

For Japan, the Iron Fist exercise is a “valuable opportunity where we can learn various techniques from the U.S. forces,” Col. Matushi Kunii, the Japanese commander of the Western Army Infantry Regiment, said at the opening ceremony last month.

For Japan, defense experts said, the shift to the more comprehensive training with the Marines is a direct response to a more assertive China. “The Japanese have been getting more serious about broadening their training,” said Christopher K. Johnson,senior adviser at the Center for Strategic and International Studies, because “the Chinese are doing their own exercises that look a lot like island-grabbing.”

He pointed to recent military exercises by China that Asia experts believe could be rehearsals for landing operations targeting the uninhabited islands.

And imagine, Asia experts said, if China became assertive about islands where people actually live, like Okinawa.

Some Asia experts believe that is already happening, pointing to recent talk from Chinese scholars, though not the Chinese government, about Okinawa, which the Japanese call Ryukyu.

“All of a sudden,” said Andrew Oros, an associate professor of political science at Washington College in Chestertown, Md., and a specialist on East Asia, “it’s no longer about protecting some deserted island; it’s about protecting somewhere where more than one million Japanese people live.”

A version of this article appears in print on , on Page A6 of the New York edition with the headline: In Japan’s Drill With the U.S., a Message for Beijing. Order Reprints | Today’s Paper | Subscribe